In recent years, in association with development of portable equipment such as personal computers and mobile phones, demands for batteries as power sources thereof have been increased. The batteries for used in the foregoing applications are required to be used at normal temperature as well as to have a high energy density and excellent cycle characteristics.
In order to satisfy these requirements, new active materials with high capacity have been developed in a positive electrode and a negative electrode. Among these, a simple substance, oxides or alloys of silicon (Si) or tin (Sn) capable of providing an extremely high capacity are regarded as promising as a negative electrode active material. Moreover, Li-containing composite oxides such as LiNiO2 are regarded as promising as a positive electrode active material.
However, an active material excellent in capability of absorbing and desorbing lithium expands and contracts considerably during charge and discharge. As a result, an electrode including a current collector is warped remarkably, easily causing wrinkles or cuts. In addition, space is created between the electrode and the separator, and uneven charge and discharge reactions tend to occur easily. There is a concern, therefore, that the battery may cause local deterioration in performance.
For such problems, one proposal suggests that a negative electrode be provided with space for relieving expansion stress of an active material. This proposal intends to suppress warps or waves of the negative electrode and prevent a deterioration of cycle characteristics. For example, Patent Document 1 suggests that columnar particles of silicon be formed on a current collector. Further, Patent Document 2 suggests that on a current collector, a pattern forming for orderly arranging particles of an active material capable of alloying with lithium be performed. Furthermore, Patent Documents 3 and 4 suggest that columnar particles forming a negative electrode active material be slanted against a direction normal to the surface of a current collector.
Both Patent Documents 1 and 2 relate to forming an active material in a columnar structure upright in a direction normal to a sheet-like current collector. Structured as such, much of the active material is not opposed to an active material in a counter electrode but is opposed to an exposed portion of the electrode current collector. For example, in the case where a negative electrode is of a columnar structure, lithium having been supplied from a positive electrode active material during charge is not absorbed in an active material of the negative electrode but readily precipitated on an exposed portion of the negative electrode current collector. As a result, during discharge, lithium is not efficiently desorbed from the negative electrode, causing a reduction in charge and discharge efficiency.
According to Patent Documents 3 and 4, it is possible to obtain a positive electrode or negative electrode active material layer while relieving the expansion of active material. In terms of a capacity retention rate, Patent Documents 3 and 4 are superior to Patent Documents 1 and 2.
Patent Document 5, although not relating to a method for producing a negative electrode for a lithium secondary battery, proposes a method for growing helical-shaped columnar particles. The helical-shaped columnar particles are formed on a substrate by vapor deposition. In this process, the slanted angle of the substrate is continuously changed with respect to an incident direction of vapor by rotating two axes orthogonal to each other.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2003-303586    Patent Document 2: Japanese Laid-Open Patent Publication No. 2004-127561    Patent Document 3: Japanese Laid-Open Patent Publication No. 2005-196970    Patent Document 4: Japanese Laid-Open Patent Publication No. Hei 6-187994    Patent Document 5: U.S. Patent Publication No. 5,866,204